The Guild Hall is available to Bavaria, Flanders, Genoa and Saxony from the 13th Century onwards. In addition to the normal special building functions, this can also be used as a means of short range defense. For a civ, the ability for the Guild Hall to defend itself and create military units can turn it into a sort of miniature castle. Although the Guild Hall's defense capabilities are not affected by technologies, its minimum range can be removed with Murder Holes. Plus, the Guild Hall heals units garrisoned inside at a considerably quicker rate than other buildings,
Saxony researches Chivalric Order instead of Close Order Drill. The Teutonic Order, furthermore, researches Swordsmanship instead of Militia Guilds and receives additional technologies.
Hit Points: 2400
Armour/Pierce Armour: 2/8
Garrison Capacity: 15 civil and military units (not siege)
Special: Heals units at faster rate than other buildings. +2 vs. spearmen.
Cost: 200 wood, 125 stone
Civ specific advantages/team bonuses
Saxony: +500 hit points (team bonus)
Wales: +20% faster training (team bonus)
Popular Assembly (15th)
Alchemy (15th, Teutonic Order)
Liv Auxiliary (14th, Teutonic Order)
Lett Auxiliary (14th, Teutonic Order)
Handgunner (15th, Flanders, Genoa)
The inability for the Guild Hall to upgrade its attack and range stems from the fact that is is a regular, class 3 building (and that the Assembly Hall, which cannot attack, uses the same unit ID, 110). One can fix this by simply reclassifying it as a class 52 tower (for the four civs that receive it and also Poland, which was formerly Lorraine, a Guild Hall civ as well). However, the built-in CPU may not build Guild Halls for the purpose of defence anyway, so it's best to leave it as it is for proper gameplay. (The CPU will build one Guild Hall only, similiar to how Princely Court civs will only build one of those. Assembly Hall civs, however, build multiples of those.)